A child (blue box) develops a currently incurable genetic disease. Scientists who wish to find a specific treatment or a means of preventing the disease in other children must trace it to its cause: a faulty gene.

Various clues, such as a visibly missing piece of a chromosome, may reveal the gene's rough location on a chromosome. When there are no such clues, researchers look for "markers" of the disease by comparing the disease of the stricken child to that of parents, relatives, and persons in other families. Eventually they find out which chromosome carries the defective gene and establish the gene's general location between known markers.

RESULT: Scientists may be able to diagnose the disease prenatally by following the inheritance of markers in an affected family. They may also recognize healthy carriers of the faulty gene (light blue boxes). The family shown here has a recessive disease that defective gene that develops only when a child inherits the defective gene from both parents.

"Walking" or "jumping" toward the gene, scientists used to create a chain of overlapping segments of DNA in the space between the flanking markers. One of these segments had to contain the faulty gene. Now that the entire human genome has been sequenced, any desired fragment of DNA is available from a genetic bank.

A mutated CF sequence

A normal CF sequence

Some letters are hidden behind the DNA twists.

Zeroing in on the faulty gene, scientists analyze each segment. Is it different from normal DNA? Finally they find the guilty gene and determine the error in its sequence of bases. The usual error in the CF gene is a deletion of three DNA bases (top) out of a total 250,000.

RESULT: Scientists can test for the disease directly in patients (blue box) and prenatally. They can identify healthy carriers of the faulty gene (light blue boxes) in the general population—not just among members of an afflicted family. They can study the disease process in cultured cells and in animals, with a view to developing new treatments.

— Maya Pines

Illustrations: Stansbury Ronsaville Wood, Inc.

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